1. Field of the Invention
The present invention relates to data communication apparatuses for information tools, and more particularly, to a communication apparatus using light such as infrared rays as a medium.
2. Description of the Background Art
The need of portable information tools such as electronic organizers or the like seems insatiable. The demand for wireless communication features incorporated in such information tools to be available in various environments and immune to external noise is great due to the portability of the information tools. As the medium of wireless communication, infrared rays are widely used since there are no statutes or regulations pertaining thereto and by virtue of their low cost.
An infrared communication system (referred to as "ASK system" hereinafter) employing ASK (Amplitude Shift Keying) as a modulation method on the basis a carrier and a subcarrier is known as a data communication system using infrared rays. Such a system is incorporated in various information tools such as electronic organizers. "Zaurus" (product name) TYPE PI-3000 and "NEWTON" (product name) TYPE PI-7000 are such exemplary apparatuses produced by Sharp Corporation.
In an ASK system, an infrared ray having a wavelength peak value in the range of 900-1050 nm is used in a physical layer. Communication is carried out using an ASK modulation where the frequency of a subcarrier is 500 kHz. In a transmission mode, a light emitting diode is energized using a rectangular wave of 500 kHz when data is 0 and the light emitting diode is deenergized when data is 1 as shown in FIG. 2.
FIG. 4 shows a protocol of a data link layer according to an ASK system with the time plotted along the abscissa. Referring to FIG. 4(a), a transmitting station sends an ENQ packet to check whether there is a receiving station to which data is to be sent. Upon receiving an ENQ packet, the receiving station transmits a SYN packet indicating that reception is allowed. Upon receiving the SYN packet, the transmitting station sends a DATA packet including data. The receiving station outputs an ACK packet when the data packet is properly received. When the transmitting station does not receive a SYN packet at an elapse of a predetermined time period after transmission of an ENQ packet, the transmitting station repeats transmission of an ENQ packet. In FIG. 4, retransmission of an ENQ packet is carried out three times.
Another infrared communication system using only a carrier on the basis of a pulse (referred to as "IrDA (Infrared Data Association) system" hereinafter) is also being realized as a data communication system.
In the IrDA system, an infrared ray having a wavelength peak value of 850 nm is used in a physical layer. As shown in FIG. 3, the light emitting diode is energized for 3/16 and deenergized for 13/16 of the time slot when data is zero at the time of transmission. When data is 1, the light emitting slot is altogether deenergized during that time slot. This transmission scheme is not dependent upon the data transfer rate.
FIG. 5 shows the protocol of a data link layer in the case of an IrDA system (referred to as "IrLAP (Infrared Link Access Protocol) hereinafter") with the time plotted along the abscissa. When either the transmitting or receiving station is establishing a connection, one station 1 monitors the media for 500 milliseconds from the start of the transmission, and sends a Discovery-XID (eXchange station IDentification)-Cmd (Command) packet if there is no traffic found during the period of 500 milliseconds. The address of that one station is included in this packet.
When that station 1 receives a packet addressed to itself during the monitor operation of the media for 500 milliseconds, the transmission is interrupted to respond to that packet. The above-described procedure can be attempted again when there is some traffic not related to its own station.
Station 1 carrying out transmission of a Discovery-XID-Cmd packet can set the number of Discovery-XID-Cmd packets (number of slots) according to the requirement of how many stations the connection is to be established with. The number of the entire slots is written in all the Discovery-XID-Cmd packets. A station 2 receiving this packet selects an appropriate slot according to a random number and returns a Discovery-XID-Rsp (Response) packet during the time slot of a Discovery-XID-Cmd packet and an End-Discovery-XID-Cmd packet indicating the end of the packet. Thus, station 1 is notified of discovery of the other party station (station 2) (corresponding to "discovery" in FIG. 5). The address of the discovered other party station is included in this packet. When the other party station is not discovered, station 1 retransmits multiple Discovery-XID-Cmd packets.
Following the discovery of station 2, station 1 transmits an SNRM (Set Normal Response Mode)-Command-Frame packet. Station 2 receiving this SNRM-Command-Frame packet responds by sending an SNRM-Command-Frame packet, whereby the operation of establishing a connection is completed (corresponds to "connect" in FIG. 5).
Following the establishment of a connection between stations 1 and 2, station 1 sends a Frame packet to station 2 to transmit data (corresponds to "information exchange" in FIG. 5).
Lastly, station 1 attempting to disconnect the connection sends an Unsequenced-Cmd packet. Station 2 receiving this packet returns an Unsequenced-Rsp packet, whereby the communication ends (corresponds to "disconnect" in FIG. 5).
It should be noted that the above-described protocol is a typical example and is subject to modifications except for the "discovery" process.
The ASK system is dedicated to a one-to-one communication. In contrast, the IrDA system allows a connection to be established with a plurality of stations at the same time i.e. "a one-to-multiple" communication. Therefore, a "discovery" procedure is required before data is transmitted in the IrDA system. In the ASK system, there is no packet exchange corresponding to "discovery" in the IrDA system since it is not necessary to identify how many stations there are as the other party. The packet exchange of packets ENQ and SYN of the ASK system corresponds to the "connect" in the IrDA system.
It is to be noted that in the ASK system, the procedure of a transmitting station and the procedure of a receiving station in establishing a connection is definitely divided. More specifically, the station transmitting data sends an ENQ packet, and the receiving station sends a SYN packet. In contrast, the role of the transmitting station and the receiving station is not divided even after a connection is established in the IrDA system.
Although the ASK system and IrDA system are both infrared communication systems, they were planned independently of each other. Therefore, there was no compatibility between the protocols of a physical layer and a data link layer. This means that one of the ASK and IrDA systems becomes the noise of the other system.
Even in the case where a circuit is incorporated in which transmission and reception of both systems are allowed, it was necessary to settle the method before a transmission/receiving operation since there is difference in protocol therebetween. It was necessary to specify by which of the systems of ASK and IrDA the communication is to be carried out, prior to the transmission/receiving operation in both the transmission apparatus and the reception apparatus. There is also a possibility of communication being disabled due to erroneous specification of a system. A novice user may not be aware of this cause, and assume that a malfunction has occurred or that he/she is out of range of the allowable receiving region.
Furthermore, the ASK system and the IrDA system have different transfer rates. The upper limit of the transfer rate of the ASK system is defined as 9600 bps (bit per second), whereas the upper limit of the transfer rate of the IrDA system is defined as 115.2 kbps. It is desirable that the system of a higher transfer rate is selected automatically when both of the ASK system and the IrDA system are allowed between two stations.
There is also a problem that frequency conversion cannot be carried out easily when the communication media is an infrared ray in contrast to the case where an radio frequency is used. This is because a light emitting diode or a semiconductor laser is used as the transmitting unit of infrared ray and a photodiode is used in the receiving unit. Therefore, the wavelength that can be used for transmission and reception can not be changed.